Conventional battery pack management systems in electric or hybrid-electric vehicles perform occasional charge-balancing operations to equalize the open-circuit voltages (OCV) of constituent blocks of battery cells within the vehicle's battery pack, theoretically normalizing the performance of the battery cells and thus improving the performance of the pack as a whole.
During pack charging and discharging operations, battery cell voltages increase and decrease, respectively, relative to their open-circuit voltages. Because the voltage across any individual battery cell must generally be maintained within specified operating limits to avoid performance degradation (e.g., reduction of the cell's operating life and/or charge storage capacity) and unsafe operation (e.g., overheating which could cause the cell to catch fire), the level of current delivered to or drawn from the battery pack is typically limited to ensure that all cells stay within operating limits.
Unfortunately, despite OCV balancing, not all battery cells exhibit the same voltage change in response to a given charging/discharging current. Consequently, the battery pack tends to suffer from a “weakest link” effect in which the cell exhibiting the worst-case (i.e., largest) voltage change in response to a given charging/discharging current establishes the maximum current that the battery pack can withstand, thus constraining the charging/discharging performance of the battery pack to that of the least healthy cell.